Response of seasonally anoestrous ewes to small-dose multiple injections of Gn-RH with and without progesterone pretreatment

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Abstract
Four groups, each of 5 seasonally anestrous ewes, were treated i.v. with small doses (75, 125, 250 or 500 ng) of GnRH [gonadotropin releasing hormone] at 2 h intervals for 48 h. A further 15 ewes received 14 days pretreatment with progesterone and then the 250 ng GnRH treatment. GnRH injections induced an episodic pattern of LH [lutropin] secretion which differed significantly for the doses of GnRH used. A preovulatory LH surge occurred in all but 1 of the ewes during the period of GnRH treatment. Ovulation occurred in all 15 ewes pretreated with progesterone and in 19 of 20 ewes treated with GnRH alone. Although normal luteal function occurred in all ewes pretreated with progesterone, it was present in only 5 of the 20 ewes treated with GnRH alone. Estrus, as shown by mating, occurred at a mean time of 34.7 .+-. 2.6 h after the start of GnRH treatment only in those ewes receiving progesterone pretreatment. Progesterone pretreatment has a marked effect on the ability of small doses of GnRH to induce ovulation and normal luteal function in seasonally anestrous ewes.

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